1. Field of the Invention
The present invention relates to a hologram manufacturing method for forming sharp interference fringes and a related exposure apparatus.
2. Related Art
Overhead projectors and slide projectors are commonly used for displaying images for numerous viewers gathering at the same place. These projectors mandatorily require screens. The screens preferably used for these projectors are generally the ones having the capability of displaying images brightly as well as reflecting particular light efficiently. To satisfy these requirements, some holographic screens have been proposed, for example, in Japanese Patent No. 5-85910, Unexamined Japanese Patent Application No. 5-333435, and Unexamined Japanese Patent Application No. 5-5953. According to these prior arts, a light diffusing member is recorded on a hologram by utilizing the selectivity of wavelength of the hologram.
FIG. 50 discloses a conventional exposure apparatus for Fresnel's hologram screens. A coherent (laser) beam 30 is emitted from a light source 91 and then separated into two beams by a half mirror 911. One of the two beams, i.e., a beam reflected by the half mirror 911, is converted into a reference beam 31 with an expanded width by a divergent lens 913. The other of the two beams, i.e., a beam penetrating the half mirror 911, is reflected by a reflection mirror 912 and then diverged by a divergent lens 914. Then, the diverged beam passes through a diffusion member 92 and is converted into a scattered beam serving as an object beam 32. Thus, interference fringes are formed on a photosensitive element 81.
However, according to the arrangement of this conventional exposure apparatus, each beam advances a relatively long optical path after being separated by the half mirror 911. This is disadvantageous in that there is a possibility of causing a significant phase difference between two beams 31 and 32 due to fluctuations of the intervening medium (i.e., air) residing along the optical paths. Such fluctuations of intervening medium possibly disturb the interference fringes formed on the photosensitive element 81. As a result, the sharpness of interference fringes will be deteriorated, and the diffraction efficiency of the hologram is lowered.
FIG. 51 discloses another conventional exposure apparatus for Fresnel's hologram screens. This conventional exposure apparatus is characterized by the photosensitive element 81 and the diffusion member 92 connected hermetically with refractive-index-adjusting liquid, such as silicon oil, kept between them. The coherent beam 30 is not separated by using a separating means such as a half mirror. The reference beam 31 is first irradiated to the diffusion member 92. Diffused beams are then introduced into the photosensitive element 81. According to this arrangement, reference beam 31 and object beam 32 cannot be discriminated clearly. This is disadvantageous in that a resultant holographic screen cannot have an intended directionality, and the transparency of the hologram is not satisfactory. Furthermore, the interference fringes formed on the photosensitive element may be disturbed due to leak of the refractive-index-adjusting liquid.
Unexamined Japanese Patent Application No. 5-232854 discloses an exposure apparatus similar to FIG. 51 but different in that the diffusion member 92 is replaced by a master hologram recording a diffusion member. In this case, there is a problem that, if noises are involved in the master hologram, the same noises are recorded in the holographic screen.